1. Field of the Invention
This invention relates to the production of homopolymers and copolymers of vinyl chloride having reduced grain size obtained by the bulk polymerization of the monomer or monomers by a two stage process more fully described in British Pat. No. 1,047,489. Such fine grain homopolymers and copolymers of 20 microns-50 microns grain size and narrow grain size distribution are useful in the preparation of plastisols and organosols in applications where bulk polymerized polymers were previously unsuited because of excessive grain size, irregular shape and wide grain size distribution. Heretofore, average grain sizes in the range of 80-150 microns were produced by prior art bulk polymerization and only by the more costly suspension or emulsion polymerization processes were fine grain size polymers obtainable.
2. Description of the Prior Art
According to British Pat. No. 1,047,489, it has been found that raising the speed of agitation during the first stage of polymerization has the effect of increasing the apparent specific gravity and contracting the distribution of grain sizes in the resin finally obtained. However, a compromise must be reached between high speed agitation during polymerization and reduction of the encrustation of the polymerization vessel during the polymerization. Thus the process described in British Pat. No. 1,047,489 also involves the use of slow speed agitation which is maintained during the second stage of the polymerization process; the agitator passes close to the walls of the polymerization vessel so as to minimize encrustation of the wall with polymer.
Heckmaier et al., U.S. Pat. No. 3,151,103 teaches production of a vinyl chloride homopolymer or copolymer using a bulk polymerization technique involving wherein agitation is accomplished by the use of a moving polymerization vessel, such as an autoclave, containing freely movable, indifferent bodies such as balls or rods of inert material, i.e. on movement of the vessel, the movable bodies agitate the polymerization mass.
The reference process is primarily directed to improving the electrical properties of the polymer product but also accomplishes reduction in polymer particle size. The reference process effects polymerization in the presence of certain metallic oxides and hydroxides, inter alia; silica, e.g. fumed silica, calcium oxide or hydroxide. The products of Examples 1-4 of the reference are reported to be reduced in grain size as indicated by greater passage through a 0.2 millimeter, i.e. 200 micron, sieve. Example 1 of the reference discloses that in polymerization in the absence of calcium oxide, the proportion of grains passing through a 0.2 millimeter sieve is 45% and that a total of 60% of the product grains pass through such a sieve when 0.5% of calcium oxide is present during the polymerization. This reduction in particle size amounts to the passage of only 15% more polyvinyl chloride particles through the sieve where calcium oxide is used than where no calcium oxide is used in the control. Similar results are given in the reference in Examples 2, 3 and 4 where the metallic oxide or hydroxide used results respectively in a total of 67% grains passing through the sieve or an increase of 22%, a total of 67.5% grain passage or an increase in 22.5% and a total of 63% grain passage amounting to an increase of only 18%. The highest amount of grain passage through the 0.2 millimeter sieve obtained in the reference examples is a total of about 67% of the polyvinyl chloride product, but there is no explicit disclosure of the average particle size of the product. The percentage of grain passage obtained over and above a control which shows a grain passage of 45% ranges from 15% to 22.5% with the fumed silica as the oxide additive in Example 4 providing a mere 18% increase over the 45% grain passage of the control for a total of 63% total grain passage through the 0.2 millimeter sieve. The minimum average product grain size is thus indicated to be only slightly under 200 microns. The indicated particle size of the reference product is thus substantially greater than the maximum average particle size obtained by the invention as hereinafter described
The reference process is also disadvantageous in its requirement to use an agitation technique that is both impractical and inconvenient, especially for large scale commercial polymerization. The use of freely movable bodies as agitation bodies propelled by movement of the entire reactor or reaction zone according to the U.S. Pat. No. 3,151,103 process entails a costly outlay of power to maintain agitation of the polymerization mass as compared to agitation achieved by conventional agitation means in a conventional stationary reaction zone. Moreover the constant impact of the agitation bodies on the walls of the reactor in the reference process would necessitate use of a reactor specially constructed for strength to withstand such impact over a prolonged period.
By the method of the invention polymers prepared in the presence of fumed silica having unexpectedly smaller average particle sizes than those of U.S. Pat. No. 3,151,103 are obtained while avoiding the costly, inconvenient and impractical moving reaction zone with freely movable agitation bodies required by the reference process to obtain a small particle size product.